Listeria monocytogenes is a very dangerous food-borne pathogen with a high mortality rate (20 - 30%). A major reason why L. monocytogenes is dangerous is because it can survive the stressful environments encountered during its infection cycle within the gastrointestinal tract, including that of high bile salt environments encountered in the small intestine and gallbladder. The ability of bacteria to establish gastrointestinal infections is directly related to their ability to resist the bactericidal properties of bile. However, very little is understood regarding the actual mechanisms of resistance, especially as they relate to gram-positive bacteria under physiologically relevant conditions. The long-term goal is to determine the mechanisms that allow for the proliferation of bacteria in the gastrointestinal tract. The specific goal of this proposal is to define the mechanism(s) utilized by L. monocytogenes to grow in bile. The rationale for this goal is that identifying the mechanism(s) of bile resistance in L. monocytogenes will lead to advancements in therapies to combat gastrointestinal infections. The central hypothesis of this proposal is that resistance of L. monocytogenes to bile is dependent upon the efficient expression of stress response mechanisms to maintain the integrity of the DNA and the cell membrane. This hypothesis will be tested by conducting two specific aims. Aim 1 entails determining the type of damage bile salts induce in L. monocytogenes under anaerobic and microaerobic conditions using in vitro and ex vivo studies. Aim 2 entails determining the role of the general stress response and the SOS response of L. monocytogenes in bile resistance by analyzing the genomic expression and physiological changes of mutants deficient in general stress or SOS response. This project will determine how bile salts induce damage in bacteria and the role that the general stress and SOS response systems have in conferring resistance to bile salts under anaerobic and microaerobic conditions. The data that will be obtained from this project are needed in order to advance our understanding of how pathogens establish infections and to develop novel therapies against enteric diseases.